Design of low power consumption LED backlight source for LCD TV

1 Introduction

Since the EU implemented the RoHS standard to eliminate lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ethers in electronic products sold in EU member states, cold cathode fluorescent tubes (CCFLs) with mercury as the main component are destined to gradually withdraw from the stage of history. At the same time, the improvement of green and environmentally friendly LED brightness has greatly promoted the development of LED backlight technology. With the advantages of wide color gamut, low operating voltage and short response time, LED backlight has shown a trend of replacing CCFL backlight. Under the theme of low carbon and environmental protection today, the competitive advantage of low-power LED backlight is more obvious.

How to achieve low power consumption is one of the main research directions of LED backlight. LED backlight is mainly composed of Back Cover, LGP, film material, Mold Frame, LED Bar, BezEL and Panel, and its energy consumption is mainly reflected in the LED Bar. By rationally designing the backlight structure, selecting suitable film materials, reducing the number of LEDs, and improving the utilization efficiency of LEDs, the power consumption of LED backlight can be greatly reduced. This paper designs a 118cm (47in) LE backlight source. Through the design of the backlight structure and the screening of film materials, the number of LEDs is reduced while ensuring the optical characteristics of the backlight brightness and uniformity, and the power consumption of the backlight is controlled at a low level.

2 LED backlight design

LED backlights are currently divided into two main types: direct-lit and edge-lit. The so-called direct-lit type is to design LED lights on the entire back of the backlight. It is usually used in large-sized backlights. Its advantages are good brightness and uniformity, but its disadvantages are that it uses a large number of LEDs and has obvious heat generation. The edge-lit type is to design LED lights at the edge of the backlight. It is usually used in small-sized backlights. Its advantages are that it uses a small number of LEDs and has better heat dissipation. However, with the improvement of LED brightness and the optimization of backlight structure, the development trend of large-sized LED backlights will gradually change from direct-lit to four-side edge-lit, then to two-side edge-lit, and finally to one-side edge-lit. Although the brightness and uniformity of direct-lit backlights are better than those of edge-lit backlights, the power consumption of edge-lit backlights is much less than that of direct-lit backlights. The 118cm (47in) LED backlight designed in this paper is an upper and lower side-lit type. There are two LED bars on each side, and each LED bar has 60 5630-LEDs. Its structure is shown in Figure 1 (the panel transmittance is set as a constant in this article). The brightness of 5630-LED was 571m/W in January 2010, and it has increased to 70-751m/W in the third quarter of 2010. It is expected to increase to 83-851m/W in the third quarter of 2011. As the luminous efficiency of LED increases, energy consumption will further decrease.

Figure 1 Structure diagram of edge-lit LED Bar

2.1 Membrane material screening

The choice of film material directly determines the performance of LED backlight.

In order to use as few LEDs as possible while maintaining good optical properties, the LED backlight designed in this paper uses 1 layer of diffusion film, 2 layers of prism film and 1 layer of double-layer brightness enhancement film (DBEF). The film material parameters are shown in Table 1. The diffusion film can change the light path of the LGP to make the emitted light more uniform; it can also hide the dots on the LGP so that the shadow of the scattering points cannot be seen from the front, expand the viewing angle and improve the brightness. The prism film is a brightness enhancement film, which mainly uses the law of total reflection and refraction to concentrate the scattered light within a certain angle range, thereby increasing the brightness within the range. DBEF is the key film material of the LED backlight module in this paper, which can double the brightness of the entire module. Its working principle is shown in Figure 2. DBEF can reflect the light that cannot pass through the lower polarizer back to the backlight. After secondary reflection of the backlight, some of the light can be emitted through DBEF and the lower polarizer. After multiple recycling and utilization, the transmittance of light can be greatly improved.

Table 1 Membrane material parameters

Figure 2 Schematic diagram of DBEF working principle

2.2 Design of LED-related spacing

Reasonable design of backlight can improve the utilization efficiency of LED. There are three main structural parameters that affect the utilization efficiency of LED:

The distance between LEDs; the distance between the LED light-emitting surface and the LGP light-entering surface; the vertical distance between the LED light-emitting surface and the LGP light-entering surface. As shown in Figure 3. Only by taking all three into consideration can the utilization efficiency of LEDs be maximized, thereby reducing power consumption.

Figure 3 Schematic diagram of LED related spacing

2.2.1 LED spacing

Since LEDs emit light from the end face, Figure 4 shows a schematic diagram of LED end face emission. The entire LED Bar is a discontinuous light source. Therefore, when the distance between LEDs is too large, the brightness will decrease, and a hot spot phenomenon of alternating light and dark may occur; when the distance between LEDs is too close, although the brightness is greatly improved, the number of LEDs required will also increase, resulting in low utilization efficiency of LEDs and high power consumption of backlights. Figure 5 is the LED brightness distribution curve obtained from the experiment. It can be seen from the figure that as the distance between LEDs increases, the brightness decreases; when the distance between LEDs d＞4.35 mm, the hot spot phenomenon will also occur, and the greater the fluctuation of the curve, the more obvious the hot spot. At the same time, the total length of the LED Bar should be less than LGP (1067×604.), so in order to ensure the brightness of the LED and avoid the hot spot phenomenon, the distance between LEDs is taken as d＝3.35mm.

Figure 4 Schematic diagram of LED end-face light emission

Figure 5 Distance between LEDs and brightness distribution curve

2.2.2 Distance from LED light-emitting surface to LGP light-entering surface

Theoretically, the smaller the distance between the LED light-emitting surface and the LGP light-entering surface, the better, so that the light emitted by the LED can be fully utilized; but in reality, due to problems such as processing accuracy and difficulty in assembly, it is impossible to achieve zero spacing. The distance from the LED light-emitting surface to the LGP light-entering surface was tested to obtain a fitting curve of the distance between the LED and LGP and the incident efficiency, as shown in Figure 6. It can be seen that the incident efficiency decreases as the distance between the LED and the LGP increases. When the distance is less than 0.8 mm, the incident efficiency decreases slowly and is above 95%. Therefore, considering the actual processing accuracy and assembly factors, the distance from the LED light-emitting surface to the LGP light-entering surface is taken as 0.8 mm.

Figure 6 LED & LGP incident efficiency distribution curve

2.2.3 Vertical distance between LED light emitting surface and LGP light incident surface

The vertical distance between the LED light-emitting surface and the LGP light-entering surface is usually ignored, but if the distance between the two is too large, it will seriously affect the incident efficiency. Through experiments, it is found that when the distance between the two is within the range of ±0.2mm, the incident efficiency changes slowly and remains at around 95%, so it is sufficient to control the vertical distance between the LED light-emitting surface and the LGP light-entering surface within the range of ±0.2mm.

FIG. 7 shows the relationship between the distance between the two and the incident efficiency.

Figure 7 LED & LGP vertical distance incident efficiency distribution curve

3 LED sample test results

Samples were made based on the above key design parameters and the selection of film materials. Figure 8 shows a real picture of a 118cm (47in) LED backlight source. The sample was compared with similar products, and the specific test results are shown in Table 2.

The sample designed in this article achieves the goal of low power consumption while ensuring optical performance, reaching the first-level energy consumption specified in the "Energy Efficiency Limit Values ​​and Energy Efficiency Grades for Flat-Panel Televisions". The product also has certain advantages in thickness and color gamut.

Figure 8 118cm (47in) LED backlight

Table 2 Sample test results

4 Conclusion

A 118cm (47in) LED backlight was designed, using 1 layer of diffusion film, 2 layers of prism film and 1 layer of DBEF to form the backlight module.

According to the test, the relevant dimensions that affect the LED utilization efficiency were optimized: the distance between LEDs d = 3.35mm, the distance from the LED light-emitting surface to the LGP light-entering surface is 0.8mm, and the distance between the horizontal center of the LED light-emitting surface and the LGP light-entering surface is controlled to be ±0.2mm. Under the premise of ensuring the optical characteristics of the backlight, the number of LEDs was reduced as much as possible to achieve low power consumption of the backlight.